A similar error led to an engineer being supplied nitrogen during confined space work in a fuel tank at Ansett's Perth hangar some years ago.
On that occasion a (grey) nitrogen bottle was connected to the confined space breathing kit instead of a (grey) compressed air cylinder. Perth used the large compressed air bottles as their air compressor did not supply air of suitable quality for filtering to breathing standard.
Luckily, the engineer was up on a stand and only up in the tank to his waist.
When he became unconscious he fell limp to the hangar floor and was revived.
So, it can happen......
MP.